Losing weight while breastfeeding is certainly not a "myth", as someone else suggested. But clearly we all have different bodies. I had sort of the opposite problem -- I lost a lot of weight when breastfeeding (and I ate a LOT) and then gained back a lot once my daughter's feeding tapered off. Sigh. So at least that won't happen to you! Because let me tell you, it is much harder for me to lose it again now that my daughter is 3 1/2. Just keep eating all the healthful food you need while you're nursing. Do NOT deprive yourself. Your baby will still get the calories he/she needs even if you're not eating as much, but you could develop vitamin deficiencies and have problems with your teeth, fingernails, hair, skin, etc. Find some books on nursing and nutrition to find ideas for which foods pack a lot of nutrition without empty calories. And drink lots of water too because you lose it when nursing.
I remember before I was pregnant that some women on Weight Watchers (I did it online) were going to a website that did a modified version of WW for nursing moms. WW itself does not endorse doing the program while pregnant or breastfeeding, but if you are pretty well-informed about nutrition and mindful that you're getting enough, that could be one option. Just do a search online for Weight Watchers and pregnancy or nursing.
And...if you aren't already doing it, make sure you are exercising regularly. I know it's not easy with a baby and work and most likely a goofy routine, but try to make it part of your daily activities, even if that just means parking the car farther away when you run errands or on your way to work.
Good luck!